Manners & Risks
There are a few simple rules to follow to make everyone experience better in the baths. Of course there exceptions to all of these rules. Watch the other bathers around you to see what is acceptable.
Use the basin and buckets found near the entrance, called the kakari-yu 掛かり湯 in Japanese, to rinse your self off before entering the bath for the first time. If there is no kakari-yu, use a shower or take water from the baths. Take special care rinse off your groin. The point is to wash away any sweat or dirt that might be on your body before entering the baths. Even better, you can wash yourself before entering the baths. That is what I do.
Do not put your head under in the water.
If you have long hair tie it up so it doesn’t get in the water.
Do not put your towel or anything other than your body or a bucket into the water.
Make sure to wipe yourself off before entering the changing room after bathing.
Do not enter the bath with your underwear on. You have to be nude. There are some exceptions to this rule, but they are rare.
Make sure you have rinsed all the soap from your body before entering the bath.
Do not yell or talk in a loud voice.
No swimming in the baths.
No food or drink. Bottled water or tea is generally OK however. (I sometimes take in a bottle of water, but I always leave it someplace away from the baths.)
Do not run.
Do not use hair dye.
Do not wash clothing in the bath. (This was acceptable behavior a long time ago.)
Rinse off any sweat and salt from the sauna before entering the baths.
Do not use salt in the sauna unless it is provided.
Do not splash water or put salt on the heater in the sauna.
Do not bring books or newspapers into the sauna. (There are some exceptions to this rule, depends on the clientele.)
When washing be careful not to hit anyone with the spray from your shower.
After washing yourself put any bathing articles you might have on the shelfs provided, or out of the way. Do not leave your things under a shower so no one else can use it.
Do not hog the baths and be considerate of your fellow bathers. (Especially in places like Nada Onsen Rokkomichi where the one gensen bath can only fit 2-3 people and is quite popular.)
Be considerate of others when brining children who have not yet been toilet-trained into the baths.
Women who are menstruating should be considerate about entering the baths.
A common fear I hear from people who have not yet been to a sento or onsen is about the possible health risks. Thankfully the biggest risk is catching athlete's foot. In the 1990's there was a fatal outbreak of Legionnaires Disease which led to a change in the laws concerning the use of recycled water in onsen and stipulating that that water must be treated before being returned to the baths. There are still however cases of Legionnaires Disease being contracted, however it appears to be rare. Most baths are treated in some manner, the most common being chlorine.
Going from the super hot bath or sauna into a cold water bath will raise the heart rate. While this is said to be good for your health, it can also be dangerous if done to an extreme. I have often gotten light headed after changing between hot and cold baths too fast. For the elderly or people with heart conditions this might be more dangerous. Also, staying in a bath, hot or cold, for too long can be dangerous.
Floors of the bathing areas are typically made tile and might be slippery. Be careful of tripping and falling. Some springs are so opaque that you cannot see the bottom of the baths. In these places be careful of hidden stairs and other objects like that might be hidden under the water. Most of these obstructions will be marked by signs, but it is important to be careful.
Certain baths like electric baths, radon baths and acidic baths have special concerns. Electric baths should be avoided by people with heart issues. Radon baths are radioactive, and children or expecting mothers are advised to avoid them. Everyone else should use their own judgement in entering them. Strongly acidic baths might cause reactions for people with fair skin.
Bathers with young children should be aware of the risks of drowning around any body of water no matter how shallow.
Up until 2014 it was thought that pregnant women should not bathe in natural hot springs, especially at the beginning and end of the pregnancy. This has been deemed without any scientific basis. Old signs saying pregnant women (妊娠中 or 妊婦) should not enter are still very common.
I am not a doctor and only presenting this information as I understand it from my research and personal experience. If you have any concerns about your health and hot springs, you should consult a doctor.